1. Field of the Invention
The present invention relates to a knock detection device for an internal combustion engine, and more particularly to a device for detecting engine knock based on an ion current inside an combustion chamber.
2. Description of the Related Art
In a gasoline engine, the air/fuel mixture in the vicinity of a spark plug is ignited by the spark produced at the spark plug, and gasoline combustion takes place with the ignited flame propagating throughout the entire air/fuel mixture. One abnormal combustion phenomenon that can occur at this time is knocking. Knocking is a condition in which unburned gases self-ignite before the flame front arrives, by an abnormally rapid rise in pressure during the flame propagation. When knock occurs, combustion gases oscillate, allowing heat to propagate more freely, and in some cases, engine damage may result. Knocking is closely related to ignition timing; as the ignition timing is advanced, maximum combustion pressure increases, increasing the tendency to knock.
On the other hand, it is desirable to increase the compression ratio in order to increase thermal efficiency and reduce fuel consumption. To achieve this, it is practiced, as part of ignition timing control, to advance the ignition timing up to the limit where knock is about to occur while detecting the occurrence of knock. Previously, in this kind of knock detection method, it was common practice to detect knock-induced vibrations using a vibration sensor attached to the cylinder block or like part, but in recent years, a knock detection method has been proposed that utilizes the change that occurs in an ion current inside a cylinder when knock occurs.
More specifically, when a spark is produced at the spark plug and air/fuel mixture burns in the combustion chamber, the air/fuel mixture is ionized. When a voltage is applied to the spark plug while the mixture is in the ionized state, an ion current flows. The occurrence of knock can be detected by detecting and analyzing this ion current. Usually, when knock occurs, an oscillating component of 6 kHz to 7 kHz appears in the ion current. The knock detection device based on the ion current extracts this frequency component peculiar to knock by means of a filter, and judges the knocking condition based on the magnitude of that component.
For example, Japanese Unexamined Patent Publication No. 4-136485 discloses a device in which a capacitor as an ion current generating source is charged to a given voltage by the secondary current that flows when the primary current of the ignition coil is shut off, and an ion current that flows, after a spark discharge, through a closed circuit consisting of the capacitor, the secondary winding of the ignition coil, the spark plug, and a current detecting resistor is measured. In such a device, since the secondary winding of the ignition coil (the secondary coil) is located in the ion current flow path, an LC resonant circuit is formed by its inductance L and the stray capacitance C associated with the coil and the spark plug. As a result, when an LC resonance current flows through the ion current path, the resonance current causes noise. To avoid such noise, the above prior art proposes that an ion current signal be masked during periods other than the period in which knock-induced oscillations appear.
However, when the ion current signal is masked during periods other than the period in which knock-induced oscillations appear, as in the above prior art, the ion current signal to which the masking has been applied will have a waveform that abruptly and discontinuously changes at the instant the mask is removed. Since such a steplike signal change has frequency components over a wide frequency range, some of the frequency components are passed unattenuated through a band-pass filter provided at a subsequent stage as a knock frequency component detection filter, and these frequency components cause noise. If such noise exists, an erroneous decision is made that knock has occurred when actually knock has not occurred.
The following problem also occurs. The period in which knock-induced oscillations appear depends on crankshaft angular position and corresponds, for example, to the position from 15.degree. to 60.degree. CA ATDC (crankshaft angle after top dead center). In a high engine rpm range, the time interval between the end of spark discharge and the start of knocking becomes short. On the other hand, the period in which LC resonance noise appears is substantially constant regardless of engine rpm, that is, a fixed period after the end of spark discharge. Therefore, at high engine rpm, the LC resonance noise period may overlap into the knock oscillation period. In that case also, an erroneous decision is made that knock has occurred when actually knock has not occurred.